The so-called Pro Sound modification for the Nintendo Game Boy bypasses some audio circuitry normally used for the headphone jack, resulting in a cleaner sound.
Here are some results from a single Nintendo Game Boy, comparing its normal headphone audio output with its Pro Sound output. Same Game Boy model, same unit, same gain setting used during recording, same recording gear. The only difference was the output used. "Stopped silence" refers to having LSDJ booted up but not playing anything at all. "Music" refers to music. "Played silence" refers to a chain and a phrase with nothing in them, but still having the LSDJ tracking actually playing. Note the difference in the Pro Sound output when comparing the stopped silence to the played silence.
- prosound: -63.9 dbFS
- normal: - 61.1 dbFS
- prosound: -2.7 dBfs
- normal: -15.8 dBfs
- prosound: -53.5 dBfs
- normal: - 63.9
Here are some sonogram images. Note how there is no increase in bass levels (or at least that I can really notice).